This invention relates to abrasive grain made from a sol-gel alumina that has improved abrasive qualities both in bonded and in coated applications.
In recent years there has been a good deal of interest in the development of sol-gel alumina abrasives. These are made by forming a sol of aluminum oxide monohydrate, (boehmite), gelling the sol, drying it and then firing the dried material to give alpha alumina. This has the advantage, inter alia, of producing a finer crystalline structure which correlates with improved abrasive performance. Such a basic process was described for example in U.S. Pat. No. 4,314,827. The crystal size within such products is generally below about 10 microns. Later it was found that this crystal size could be reduced by about an order of magnitude, with corresponding improvements in the abrasive qualities, if the sol-gel was seeded with a material that promotes the growth of the alpha alumina final product. This produced the "seeded sol-gel" process improvement that has had a great deal of commercial success in recent years and is described in U.S. Pat. No. 4,623,364.
Another development that has significant potential is the invention of sol-gel alumina abrasive filaments. These are abrasive particles that have a generally uniform cross-sectional shape about an axis, with an aspect ratio of greater than 1. The aspect ratio is the axial length divided by the greatest dimension perpendicular to that axis. Such particles are preferably made by a seeded sol-gel alumina process.
As explained above such sol-gel aluminas perform at a much higher performance level than the earlier fused aluminas. One occasional problem however has been the fact that the grains are strong and insufficiently friable. A "friable" grain, as the term is used herein, describes a grain that continuously exposes new cutting edges as a result of fracture under the grinding load. Grains that lack friability perform best under the most aggressive cutting conditions and tend to leave a surface finish that is rougher than might be acceptable for some applications.
Traditionally this is overcome by grinding during the finishing stages with a finer grain abrasive and this is still effective with sol-gel alumina grains. It would be desirable however if the amount of finishing that is required could be minimized.
Another consequence of the extremely aggressive cutting action of the sol-gel alumina abrasive grains is that the advantages they bring are not so readily apparent under low pressure grinding conditions.
There is therefore a need for an abrasive sol-gel alumina abrasive grain that will perform well at relatively low pressure grinding conditions and will leave a smoother finish on the ground surface.
The present invention describes a product, and a process for making such a product, that has the very advantageous friability and is therefore suitable for low pressure grinding.